I’m using Hyprland, but tested KDE Plasma, same results.
My system: NixOS 25.05 with kernel 6.15.8-zen1.
CPU: Ryzen 5 5600G (iGPU)
Under any I/O load that is even slightly heavy (for example, downloading a torrent at around 25 MB/s), all windows begin to freeze heavily.
It would seem that the problem could be with the disk, but I have a Samsung 980 (1 TB), and did performance tests (I use encryption and compression with the btrfs zstd:4 algorithm). The speeds according to the test results were quite normal; at least it definitely shouldn’t freeze so badly when writing at 25 MB/s.